This invention relates to a system for catalytically cracking and desulfurizing hydrocarbon-containing fluids. In another aspect, the invention concerns an integrated system for cracking and desulfurizing hydrocarbon-containing fluids in a common cracking/desulfurization unit.
Fluidized catalytic cracking (FCC) units are used in the petroleum industry to convert (i.e., “crack”) high boiling point hydrocarbon feedstocks (e.g., gas oils) to more valuable hydrocarbon products (e.g., gasoline) having a lower average molecular weight and a lower average boiling point than the feedstocks from which they were derived. The conversion is normally accomplished by contacting the hydrocarbon feedstock with finely divided solid catalyst particulates at elevated temperatures. The most typical hydrocarbon feedstock treated in FCC units comprises a heavy gas oil, but on occasions such as feedstocks as light gas oils, naphthas, reduced crudes and even whole crudes are subjected to catalytic cracking to yield lower boiling hydrocarbon products.
Catalytic cracking in FCC units is generally accomplished via a cyclic process involving separate zones for catalytic reaction, stripping, and catalyst regeneration. The hydrocarbon feedstock is blended with an appropriate amount of catalyst particles to form a mixture that is then passed to a catalytic reactor, normally referred to as a riser, wherein the mixture is subjected to a temperature between about 800° F. and about 1100° F., normally between about 900° F. and 1050° F., in order to convert the feedstock into gaseous, lower boiling hydrocarbons. After these lower boiling hydrocarbons are separated from the catalyst in a suitable separator, the catalyst, now deactivated by coke deposited upon its surfaces, is passed to a stripper. Here the deactivated catalyst is contacted with a stripping fluid to remove entrained hydrocarbons. The deactivated coke-containing catalyst particles recovered from the stripper are then introduced into a regenerator where the catalyst is reactivated by combusting the coke in the presence of an oxygen-containing gas, such as air, at a temperature which normally ranges between about 1000° F. and 1500° F. The cyclic process is then completed by blending the reactivated catalyst particles with the feedstock entering the reactor of the FCC unit.
The hydrocarbon feedstock to the FCC reactor and the cracked hydrocarbon product from the FCC reactor typically contain quantities of sulfur. If the cracked hydrocarbon product (e.g., cracked-gasoline) from the FCC reactor is employed in the making of automotive fuels, combustion of such fuels in the engine of an automobile may result in a sulfur-containing combustion exhaust that can irreversibly poison the noble metal catalysts in the automobile's catalytic converter. Emissions from such poisoned catalytic converters may contain high levels of non-combusted hydrocarbons, oxides of nitrogen, and/or carbon monoxide, which, when catalyzed by sunlight, form ground level ozone, more commonly referred to as smog. Thus it is desirable for the hydrocarbon products of a FCC unit to have minimal sulfur content.
Many conventional processes exist for removing sulfur from cracked hydrocarbon products (e.g., cracked-gasoline). However, most conventional sulfur removal processes, such as hydrodesulfurization, tend to saturate olefins and aromatics in the cracked hydrocarbon product and thereby reduce its octane number (both research and motor octane number). Thus, there is a need for a process wherein desulfurization of cracked hydrocarbon products is achieved while the octane number is maintained.
Recently, an improved system for removing sulfur from hydrocarbon-containing fluids, such as cracked-gasoline and diesel fuel, has been developed. This system employs fluidizable, regenerable, bifunctional solid sorbent particulates to remove both organic and inorganic sulfur compounds from the hydrocarbon-containing fluid. The desulfurization unit comprises separate reactor, regenerator, and reducer vessels. The sorbent is circulated through the reactor, regenerator, and reducer in a substantially continuous manner. In the reactor, the bifunctional sorbent (comprising a promoter metal component and zinc oxide) is contacted with, and removes sulfur from, the hydrocarbon-containing fluid. The sulfur-loaded sorbent is then transported from the reactor to the regenerator where it is contacted with an oxygen-containing regeneration stream to thereby remove sulfur from the sorbent. The regenerated sorbent is then transported to the reducer where it is contacted with a hydrogen-containing regeneration stream to reduce the promoter metal component. After reduction, the regenerated and reduced sorbent is returned to the reactor for sulfur removal.
In conventional petroleum processing facilities, the FCC and desulfurization units are separate from one another. Thus, different reactor and regenerator vessels are used for the FCC and desulfurization units. However, the construction, operation, and maintenance expense of FCC and desulfurization units could be greatly reduced if fluid catalytic cracking and desulfurization could be carried out in the same vessels, at the same time, and under the same conditions. Such an integrated FCC/desulfurization system would be a significant contribution to the art and to the economy.